The production of nitrous oxides (hereinafter "NOx") occurs as a result of combustion at high temperatures. NOx is a notorious pollutant, and as a result, combustion devices which produce NOx are subject to ever more stringent standards for emissions of such pollutants. Accordingly, much effort is being put forth to reduce the formation of NOx in combustion devices.
One solution has been to premix the fuel with an excess of air such that the combustion occurs with local high excess air, resulting in a relatively low combustion temperature and thereby minimizing the formation of NOx. A fuel nozzle which so operates is shown in U.S. Pat. No. 5,307,634, which discloses a scroll swirler with a conical center body. This type of fuel nozzle is known as a tangential entry fuel nozzle, and comprises two offset cylindrical-arc scrolls connected to two endplates. Combustion air enters the swirler through two substantially rectangular slots formed by the offset scrolls, and exits through a combustor inlet port in one endplate and flows into the combustor. A linear array of orifices located on the outer scroll opposite the inner trailing edge injects fuel into the airflow at each inlet slot from a manifold to produce a uniform fuel air mixture before exiting into the combustor.
Premix fuel nozzles of the tangential entry type have demonstrated low emissions of NOx relative to fuel nozzles of the prior art. Unfortunately, fuel nozzles such as the one disclosed in the aforementioned patent have been shown, at certain operating conditions, to produce acoustic tones and excessive combustor pressure fluctuations which lead to deterioration of the gas turbine engine. As a result, tangential entry fuel nozzles of this type have not been incorporated into commercially available gas turbine engines.
What is needed is a method of combustion that significantly reduces the acoustic effects that lead to the excessive combustor pressure fluctuations.